Ion exchangeable li-containing glass compositions for 3-d forming

ABSTRACT

According to one embodiment, a glass article may include SiO2, Al2O3, Li2O and Na2O. The glass article may have a softening point less than or equal to about 810° C. The glass article may also have a high temperature CTE less than or equal to about 27×10−6/° C. The glass article may also be ion exchangeable such that the glass has a compressive stress greater than or equal to about 600 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO3 at a temperature in a range from about 390° C. to about 450° C. for less than or equal to approximately 15 hours.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and claims the benefit ofpriority under 35 U.S.C. § 120 of U.S. application Ser. No. 16/181,893filed on Nov. 6, 2018, which in turn, is a divisional of U.S.application Ser. No. 14/824,653 filed on Aug. 12, 2015, now patent Ser.No. 10/150,691 granted Dec. 11, 2018, which is a divisional applicationof U.S. application Ser. No. 13/938,579 filed on Jul. 10, 2013, now U.S.Pat. No. 9,139,469 granted Sep. 22, 2015, which in turn, claims thebenefit of priority under 35 U.S.C. § 119 of U.S. ProvisionalApplication Ser. No. 61/672,346 filed Jul. 17, 2012 and entitled “IonExchangeable Li-Containing Glass Compositions For 3-D Forming,” thecontents of each of which are relied upon and incorporated herein byreference in their entireties.

BACKGROUND Field

The present specification generally relates to glass compositionssuitable for use in 3-D forming applications and, more specifically, toion exchangeable, Li-containing glass compositions suitable for 3-Dforming.

Technical Background

Ion exchangeable glass compositions are widely used as cover glasses inmany electronic devices including mobile telephones, personal mediaplayers, tablet computers and the like. The cover glasses used in theseapplications are generally flat and planar. As such, the cover glassesmay be formed using conventional glass forming processes, such as downdraw processes and/or float processes.

One limiting factor in the aesthetic design of electronic devices is theability to shape the cover glasses to conform to curved and/or complexcontours. Glass compositions which are amenable to ion exchangegenerally have relatively high softening points, making the glasscompositions difficult to form into 3-D shapes using elevatedtemperature forming processes such as vacuum sagging. As a result of therelatively high softening points, the glass compositions tend to reactwith the material of the mold, sticking to the mold and/or degrading themold, even when protective coatings are applied to the mold.

Accordingly, a need exists for alternative glass compositions suitablefor use in elevated temperature 3-D forming processes and which are alsoamenable to strengthening by ion exchange processing.

SUMMARY

According to one embodiment, a glass article may include SiO₂, Al₂O₃,Li₂O and Na₂O. The glass article may have a softening point less than orequal to about 810° C. The glass article may also have a hightemperature coefficient thermal expansion (“CTE”) less than or equal toabout 27×10⁻⁶/° C. The glass article may also be ion exchangeable suchthat the glass has a compressive stress greater than or equal to about600 MPa and a depth of layer greater than or equal to about 25 μm afterion exchange in a salt bath comprising KNO₃ at about 410° C. in atemperature range from about 390° C. to about 450° C. for less than orequal to approximately 15 hours.

In another embodiment, a glass composition may include from about 65.8mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. %Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. %to about 16 mol. % Na₂O; and from about 0.8-10 mol. % of a divalentoxide, wherein the divalent oxide includes at least one of MgO and ZnO;and less than about 0.5 mol. % B₂O₃. A sum of a concentration of Al₂O₃(mol. %) and a concentration of the divalent oxide (mol. %) may begreater than 10 mol %. The glass composition may have a softening pointless than or equal to about 810° C. The glass composition may also havea high temperature coefficient thermal expansion (“CTE”) less than orequal to about 27×10⁻⁶/° C. These glass compositions may besubstantially free of ZrO₂.

In yet another embodiment, a glass composition may include from about 55mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. %Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. %to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; andfrom about 0 mol. % to about 3 mol. % K₂O. In this embodiment, R₂O is asum of a concentration of Li₂O, a concentration of Na₂O, and aconcentration of K₂O. The ratio of R₂O to a concentration of Al₂O₃ isless than or equal to about 1.5. The glass composition may have asoftening point less than or equal to about 810° C. The glasscomposition may also have a high temperature CTE less than or equal toabout 27×10⁻⁶/° C.

In yet another embodiment, a glass composition may include from about 65mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. %Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. %to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. % K₂O; fromabout 0.8 to about 10 mol. % of a divalent oxide, wherein the divalentoxide comprises at least one of MgO and ZnO; from about 0.5 mol. % toabout 2 mol. % ZrO₂; and less than about 0.5 mol. % B₂O₃, wherein: thesum of the concentration of Al₂O₃ (mol. %), and the concentration of thedivalent oxide (mol. %) is greater than about 10 mol %. The glasscomposition has a softening point less than or equal to about 810° C.;and a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

Additional features and advantages of the glass compositions describedherein will be set forth in the detailed description which follows, andin part will be readily apparent to those skilled in the art from thatdescription or recognized by practicing the embodiments describedherein, including the detailed description which follows, the claims, aswell as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the various embodiments describedherein, and together with the description serve to explain theprinciples and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically depicts the instantaneous CTE (y-axis) as a functionof temperature (x-axis) for two comparative glass compositions;

FIG. 2 graphically depicts the softening point (y-axis) as a function ofthe concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O in anexemplary glass composition;

FIG. 3 graphically depicts the HT CTE (y-axis) as a function of theconcentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O in anexemplary glass composition;

FIG. 4 graphically depicts the softening point (y-axis) as a function ofthe concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O andK₂O in an exemplary glass composition;

FIG. 5 graphically depicts the HT CTE (y-axis) as a function of theconcentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O and K₂Oin an exemplary glass composition;

FIG. 6 graphically depicts the compressive stress and DOL values plottedfor different Li₂O concentrations; and

FIG. 7 graphically depicts the concentration of potassium and sodiumions (y-axis) as a function of depth (x-axis) for an ion-exchanged glassplate formed from an exemplary glass composition containing colormodifying constituents.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of ionexchangeable glass compositions which are suitable for use with 3-Dforming processes. The glass compositions described herein generallyinclude SiO₂, Al₂O₃, Li₂O and Na₂O. The glass composition may havesoftening points less than or equal to about 810° C. The glasscomposition may also have high temperature CTEs less than or equal toabout 27×10⁻⁶/° C. The glass compositions may also be ion exchangeablesuch that the glass has a compressive stress greater than or equal toabout 650 MPa and a depth of layer greater than or equal to about 25 μmafter ion exchange in a salt bath comprising KNO₃ at about 410° C. forless than or equal to approximately 15 hours. Various embodiments of theglass compositions will be described in further detail herein withspecific reference to the appended drawings.

The term “softening point,” as used herein, refers to the temperature atwhich the viscosity of the glass composition is 1×10^(7.6) poise.

The phrase “high temperature coefficient of thermal expansion” or “HTCTE,” as used herein, refers to the coefficient of thermal expansion ofthe glass composition above the glass transition temperature of theglass composition. The HT CTE is determined by plotting theinstantaneous CTE (y-axis) as a function of the temperature (x-axis).The HT CTE is the value of the HT CTE where the slope of the CTE v.temperature curve is approximately zero following a pronounced increase(i.e., where the CTE v. temperature curve “plateaus”). The value of theHT CTE is a measure of the volume change of the glass during cooling andis an indication of the dimensional stability of the glass compositionwhen the glass is utilized in conjunction with elevated temperature 3-Dforming process including, without limitation, vacuum sagging formingprocesses.

The term “liquidus viscosity,” as used herein, refers to the shearviscosity of the glass composition at its liquidus temperature.

The term “liquidus temperature,” as used herein, refers to the highesttemperature at which devitrification occurs in the glass composition.

The term “substantially free,” when used to described the absence of aparticular oxide component in a glass composition, means that thecomponent is present in the glass composition as a contaminant in atrace amount of less than about 0.05 mol. %.

In the embodiments of the glass compositions described herein, theconcentration of constituent components (e.g., SiO₂, Al₂O₃, B₂O₃ and thelike) are given in mole percent (mol. %) on an oxide basis, unlessotherwise specified.

Conventional ion exchangeable glass compositions used as cover glassesin consumer electronic devices, generally have softening points of 840°C. or greater. Glasses with softening points in this range may bereadily formed into planar sheets using fusion forming processes.However, such glass compositions are not always amenable to elevatedtemperature forming process. Specifically, the relatively high softeningpoints of the glass compositions cause the glass compositions to reactwith the material of the mold such that the glass composition eithersticks to the mold damaging the glass and/or degrading the mold, evenwhen protective coatings are applied to the mold.

Further, attempts to improve the formability of ion exchangeable glasscompositions by decreasing the softening points of the glasscompositions have not been successful. Specifically, it has been foundthat glass compositions with lower softening points did not have therequisite dimensional stability for 3-D forming using elevatedtemperature processes such as vacuum sagging. Such glass compositionswarp upon forming as the compositions are heated and/or cooled throughthe glass transformation region.

For example, FIG. 1 graphically depicts the instantaneous CTE (y-axis)as a function of temperature (x-axis) for two comparative glasscompositions. Comparative glass A was a borosilicate glass which had asoftening point of 752° C. and an HT CTE of approximately 39×10⁻⁶/° C.While not wishing to be bound by theory, it is believed that thisrelatively high HT CTE reduced the dimensional stability of the glassupon vacuum sagging, causing the glass to warp and distort. In contrast,Comparative glass B was an aluminosilicate glass which had a softeningpoint of 837° C. and an HT CTE of approximately 23.2×10⁻⁶/° C. Whilethis glass exhibited a relatively low HT CTE, it was found that theglass composition reacted with and/or stuck to the mold during vacuumsagging, inhibiting formation. While not wishing to be bound by theory,it is believed that the inability to consistently form Comparative glassB was due, at least in part, to the relatively higher softening point ofthe glass.

The glass compositions described herein address the deficiencies ofprevious glass compositions by providing glass compositions which haverelatively low softening points, relatively low HT CTEs and havesuperior ion exchange performance relative to existing 3-D formableglass compositions.

In the embodiments described herein, the glass compositions haverelatively low softening points of less than or equal to about 810° C.In some embodiments, the softening point of the glass composition may beless than or equal to about 800° C. or even less than or equal to about790° C. In some other embodiments, the softening points may be less thanabout 750° C. The relatively low softening points of these glasscompositions facilitate readily forming the glass compositions into 3-Dshapes, such as glass articles with complex curvatures and the like,using vacuum sagging processes.

The glass compositions also have HT CTEs of less than or equal to about27×10⁻⁶/° C. In some embodiments, the HT CTE of the glass compositionmay be less than or equal to about 25×10⁻⁶/° C. or even less than orequal to about 23×10⁻⁶/° C. As noted above, the HT CTE is an indicationof the dimensional stability of the glass when the glass is utilized inconjunction with elevated temperature 3-D forming process including,without limitation, vacuum sagging processes. It has been determinedthat glasses which have HT CTEs greater than 27×10⁻⁶/° C. may warpduring and/or after elevated temperature forming processes resulting ina glass article which may not conform to dimensional tolerances.However, it has also been determined that glasses with moderately lowerHT CTEs, such as HT CTEs less than or equal to about 27×10⁻⁶/° C., aredimensionally stable during and following elevated temperature formingprocesses.

The glass compositions described herein are also amenable tostrengthening by ion exchange processes. In the embodiments describedherein, the glass compositions are able to achieve a depth of layer(DOL) of greater than or equal to about 25 μm. In some embodiments, theDOL may be greater than or equal to about 35 μm or even greater than orequal to about 45 μm. The compressive stress (CS) of the glasscomposition may be greater than or equal to about 600 MPa or evengreater than or equal to about 650 MPa. Both the compressive stress andthe DOL are determined following ion exchange strengthening in a saltbath comprising 100% KNO₃ or a salt bath comprising KNO₃ and NaNO₃ forless than or equal to approximately 15 hours at temperatures from about390° C. to about 450° C.

In order to achieve the aforementioned properties, the glasscompositions described herein generally include a combination of SiO₂,Al₂O₃, and alkali oxides such as Li₂O and/or Na₂O. In some embodiments,the glass compositions may also include one or more divalent oxides,such as MgO, ZnO, CaO or the like. The glass compositions may alsoinclude B₂O₃. In some embodiments, the glass compositions may alsocomprise K₂O in addition to Li₂O and/or Na₂O. The glass compositions mayadditionally comprise P₂O₅. The glass compositions may also comprise oneor more fining agents. The concentrations of these various constituentcomponents used to achieve glass compositions having the aforementionedproperties will be described in further detail herein.

As noted above the glass compositions described herein may include B₂O₃.In some embodiments, the concentration of B₂O₃ in the glass compositionmay be less than or equal to about 1.0 mol. % or even less than or equalto about 0.5 mol. %, including about 0 mol. % (i.e., glass compositionswhich are substantially free from B₂O₃). These glass compositions may bereferred to herein as “low boron glass compositions.” In otherembodiments, the concentration of B₂O₃ may be greater than or equal toabout 4.5 mol. %. These glass compositions may be referred to herein as“high boron glass compositions.” However, it should be understood thatthe low boron glass compositions and the high boron glass compositionsboth exhibit the relatively low softening points, relatively low HT CTEsand ion exchangeability described above.

In the embodiments of the glass compositions described herein (i.e.,both low boron glass compositions and high boron glass compositions),SiO₂ is the largest constituent of the composition and, as such, SiO₂ isthe primary constituent of the glass network. When the concentration ofSiO₂ in the glass composition is low (i.e., less than about 55 mol. %)the chemical durability of the resultant glass is low. In addition, theliquidus viscosity of the resultant glass may also be low rendering theglass unsuitable for fusion formation, such as with a fusion down drawprocess and/or a fusion lamination process. However, if theconcentration of SiO₂ in the glass composition is too high (i.e.,greater than about 75 mol. %), the formability of the glass compositionmay be diminished as higher concentrations of SiO₂ increase thedifficulty of melting the glass which, in turn, adversely impacts theformability of the glass. In the embodiments described herein, the glasscomposition generally comprises SiO₂ in a concentration greater than orequal to about 55 mol. % and less than or equal to about 75 mol. % inorder to facilitate a readily formable glass compositions.

In the low boron glass compositions, the concentration of SiO₂ may begreater than or equal to about 65 mol. % and less than or equal to about71 mol %. In some embodiments, the concentration of SiO₂ may be greaterthan or equal to about 65.8 mol. % or even about 66 mol. % and less thanor equal to about 71 mol. %. In some other embodiments, theconcentration of SiO₂ in the glass composition may be greater than orequal to about 67 mol. % and less than or equal to about 71 mol. %. Insome embodiments, the concentration of SiO₂ in the glass composition maybe greater than or equal to about 68 mol. % and less than or equal toabout 71 mol. %.

In the high boron glass compositions, the concentration of SiO₂ may begreater than or equal to about 55 mol. % and less than or equal to about68 mol. %. In some of these embodiments, the concentration of SiO₂ inthe glass composition may be greater than or equal to about 60 mol. %and less than or equal to about 65 mol. %.

The glass compositions described herein (i.e., both low boron glasscompositions and high boron glass compositions) also comprise Al₂O₃.Al₂O₃ serves as a glass network former, similar to SiO₂. Like SiO₂,Al₂O₃ increases the viscosity of the glass composition due to itsprimarily tetrahedral coordination in a glass melt formed from the glasscomposition. Al₂O₃ improves the ion-exchange performance of the glasscomposition by increasing the strain point of the glass and increasingthe diffusivity of alkali ions in the glass network. Accordingly, thepresence of Al₂O₃ improves the kinetics of the ion-exchange process andincreases the maximum compressive stress and DOL which can be obtained.In order to obtain the improvement in the kinetics of the ion-exchangeprocess, the concentration of Al₂O₃ in the glass compositions isgenerally greater than or equal to about 7 mol. %.

In the embodiments of the low boron glass compositions described herein,the concentration of Al₂O₃ in the glass compositions is generally lessthan or equal to about 12 mol. % in order to achieve a glass compositionwhich has a relatively low softening point. For example, in someembodiments, the concentration of Al₂O₃ in the glass compositions isgreater than or equal to about 7 mol. % and less than or equal to about12 mol. %. In some embodiments, the concentration of Al₂O₃ in the glasscompositions may be greater than or equal to about 8 mol. % and lessthan or equal to about 12 mol. %. In some other embodiments, theconcentration of Al₂O₃ may be greater than or equal to about 8 mol. %and less than or equal to about 11 mol. %.

In the embodiments of the high boron glass compositions describedherein, the concentration of Al₂O₃ in the glass compositions isgenerally less than or equal to about 15 mol. % in order to achieve aglass composition which has a relatively low softening point. Forexample, in some embodiments, the concentration of Al₂O₃ in the glasscompositions is greater than or equal to about 9 mol. % and less than orequal to about 15 mol. %. In some embodiments, the concentration ofAl₂O₃ in the glass compositions may be greater than or equal to about 11mol. % and less than or equal to about 14 mol. %.

The glass compositions described herein (i.e., both low boron glassboron glass compositions and high boron glass compositions) also includealkali oxide R₂O where R is at least one of Li, Na, K or combinationsthereof. The alkali oxides lower the melting temperature and theliquidus temperature of the glass, thereby improving the formability ofthe glass composition. Additions of Li₂O generally decrease thesoftening point of the glass. The amount of Li₂O in the glasscomposition can be adjusted to improve the reaction kinetics of the ionexchange process. Specifically, if a faster ion exchange process isdesired, the concentration of Li₂O in the glass compositions may beoptionally decreased to less than about 5 mol. %, such as from greaterthan or equal to about 1 mol. % to less than or equal to about 5 mol. %,or even from greater than or equal to about 2 mol. % to less than orequal to about 5 mol. %, in order to increase the ion exchange ratewhile also decreasing the softening point of the glass.

In the embodiments of the low boron glass compositions described herein,Li₂O is generally added to the glass compositions to decrease thesoftening point of the glass. The concentration of Li₂O in the glasscompositions is generally greater than or equal to about 1 mol. % inorder to achieve a glass composition which has a relatively lowsoftening point. For example, in some embodiments, the concentration ofLi₂O in the glass compositions is greater than or equal to about 1 mol.% and less than or equal to about 9 mol. %. In some embodiments, theconcentration of Li₂O in the glass compositions may be greater than orequal to about 1 mol. % and less than or equal to about 7 mol. %. Inembodiments where a faster ion exchange time is desired for a giventemperature, the concentration of Li₂O may be greater than or equal toabout 1 mol. % and less than or equal to about 5 mol. %, or even greaterthan or equal to about 2 mol. % and less than or equal to about 5 mol.%. In some other embodiments, the concentration of Li₂O may be greaterthan or equal to about 2 mol. % and less than or equal to about 3.5 mol.%.

In the embodiments of the high boron glass compositions describedherein, the concentration of Li₂O in the glass compositions is generallygreater than or equal to about 1 mol. % in order to achieve a glasscomposition which has a relatively low softening point. For example, insome embodiments, the concentration of Li₂O in the glass compositions isgreater than or equal to about 1 mol. % and less than or equal to about7 mol. %. In embodiments where a faster ion exchange time is desired fora given temperature, the concentration of Li₂O may be greater than orequal to about 1 mol. % and less than or equal to about 5 mol. %, oreven greater than or equal to about 2 mol. % and less than or equal toabout 5 mol. %.

Additions of Na₂O in the glass compositions facilitate ion exchangestrengthening the glass compositions. Specifically, the smaller Na⁺ ionsin the resultant glass network can be exchanged for larger K⁺ ions inthe ion exchange salt bath. If the Na₂O concentration in the glasscomposition is too low, the resultant depth of layer after ion exchangeis too low. However, if the Na₂O concentration in the glass compositionin too high, the HT CTE of the glass composition increases. In theembodiments described herein, the Na₂O is present in the glasscompositions in an amount from about 3 mol. % to about 16 mol. %.

In the embodiments of the low boron glass compositions described herein,the concentration of Na₂O in the glass compositions is generally lessthan or equal to about 16 mol. % in order to maintain a relatively lowHT CTE. For example, in some embodiments, the concentration of Na₂O inthe glass compositions is greater than or equal to about 6 mol. % andless than or equal to about 16 mol. %. In some embodiments, theconcentration of Na₂O in the glass compositions may be greater than orequal to about 8 mol. % and less than or equal to about 16 mol. %. Insome other embodiments, the concentration of Na₂O in the glasscompositions may be greater than or equal to about 10 mol. % and lessthan or equal to about 16 mol. %. In still other embodiments, theconcentration of Na₂O in the glass compositions may be greater than orequal to about 12 mol. % and less than or equal to about 15 mol. %.

In the embodiments of the high boron glass compositions describedherein, the concentration of Na₂O in the glass compositions is generallyless than or equal to about 12 mol. % in order to maintain a low alkalito alumina ratio and resultant relatively low HT CTE. For example, insome embodiments, the concentration of Na₂O in the glass compositions isgreater than or equal to about 3 mol. % and less than or equal to about12 mol. %. In some embodiments, the concentration of Na₂O in the glasscompositions may be greater than or equal to about 8 mol. % and lessthan or equal to about 12 mol. %.

In some embodiments, the glass compositions described herein mayoptionally include the alkali oxide K₂O. K₂O is generally added to theglass compositions to improve ion exchange performance. Specifically,K₂O may be added to the glass compositions in order to achieve thedesired compressive stress and DOL. In the embodiments described herein,K₂O, when included, is present in the glass compositions in an amountless than or equal to about 3.0 mol. %.

In the embodiments of the low boron glass compositions described herein,the concentration of K₂O in the glass compositions is generally greaterthan or equal to about 0 mol. %. For example, in some embodiments, theconcentration of K₂O in the glass compositions is greater than or equalto about 0 mol. % and less than or equal to about 5 mol. %. In someembodiments, the concentration of K₂O in the glass compositions may begreater than or equal to about 0 mol. % and less than or equal to about3 mol. %. In some other embodiments, the concentration of K₂O in theglass compositions may be greater than or equal to about 0 mol. % andless than or equal to about 2 mol. %. In some other embodiments, theconcentration of K₂O in the glass compositions may be less than or equalto about 1 mol. % or even less than or equal to about 0.5 mol. %. Insome embodiments, the low boron glass compositions are substantiallyfree of K₂O.

In the embodiments of the high boron glass compositions describedherein, the concentration of K₂O in the glass compositions is generallygreater than or equal to about 0 mol. %. For example, in someembodiments, the concentration of K₂O in the glass compositions isgreater than or equal to about 0 mol. % and less than or equal to about3 mol. %. In some embodiments, the concentration of K₂O in the glasscompositions may be greater than or equal to about 0 mol. % and lessthan or equal to about 2 mol. %. In some other embodiments, theconcentration of K₂O in the glass compositions may be less than or equalto about 1 mol. % or even less than or equal to about 0.5 mol. %. Insome embodiments, the high boron glass compositions are substantiallyfree of K₂O.

Further, in the embodiments of the high boron glass compositiondescribed herein, the ratio of R₂O to the concentration of Al₂O₃ isgenerally less than or equal to about 1.15 or even 1.1, where R₂O is thesum of the concentrations of Na₂O, Li₂O and K₂O. In some of theseembodiments, the ratio of R₂O to the concentration of Al₂O₃ is generallygreater than or equal to about 0.9. In some embodiments, the ratioR₂O:Al₂O₃ is less than or equal to about 1.1 and greater than or equalto about 0.9. In some embodiments, the ratio R₂O:Al₂O₃ is less than orequal to about 1.1 and greater than or equal to about 1.0. In some otherembodiments, the HT CTE is less than or equal to about 1.0 and greaterthan or equal to 0.9. Maintaining the ratio R₂O:Al₂O₃ at less than about1.15 or even 1.1 in high boron glass composition generally lowers the HTCTE to less than or equal to about 27×10-6/° C. In particular, in theembodiments of the glass compositions described herein, the sum of theconcentration of alkali oxide constituents is balanced against theconcentration of Al₂O₃. This balance produces several desirablecharacteristics in the resulting glass. Specifically, Al₂O₃ utilizesalkali metals, such as the potassium, lithium and sodium, for chargestabilization. If excess alkali is present in the glass composition(i.e., R₂O:Al₂O₃ is greater than or equal to 1.15 or even 1.1), theexcess alkali in the glass composition interacts with the boron in theglass composition and converts the boron from its standard trigonal(threefold-coordinated) configuration into a tetrahedral(fourfold-coordinated) configuration. The change in the coordination ofthe boron from trigonal to tetrahedral increases the HT CTE of theglass. Accordingly, by maintaining the ratio of R₂O to Al₂O₃ at lessthan or equal to about 1.15 or even less than or equal to about 1.1prevents the boron in the glass from assuming a tetrahedral coordinationin the glass and thereby promotes a relatively low HT CTE.

In some embodiments described herein, the glass compositions may includeB₂O₃. Like SiO₂ and Al₂O₃, B₂O₃ contributes to the formation of theglass network. Conventionally, B₂O₃ is added to a glass composition inorder to decrease the viscosity of the glass composition. In general,B₂O₃ acts as a flux which may be utilized to form glass compositionswith low softening points. However, the presence of B₂O₃ significantlyincreases the HT CTE when excess alkali oxides are present which are notassociated with Al₂O₃. However, if the concentration of alkali oxides isbalanced against the concentration of Al₂O₃, higher concentrations ofboron oxide do not significantly impact the HT CTE of the glasscomposition. Accordingly, the glass compositions described herein maycontain a low concentration of B₂O₃ (i.e., the “low boron glasscompositions”) or a high concentration of B₂O₃ (i.e., the “high boronglass compositions”) in order to achieve a glass composition having thedesired properties.

In the low boron glass compositions, B₂O₃ is generally present in theglass compositions in an amount less than or equal to about 1 mol. % inorder to minimize the increase in HT CTE by limiting the interactionbetween B₂O₃ and excess alkali oxides. For example, in some embodiments,B₂O₃ is present in the glass compositions in a concentration greaterthan or equal to about 0 mol. % and less than or equal to about 1 mol.%. In other embodiments described herein, B₂O₃ is present in the glasscompositions in a concentration of less than about 0.5 mol. %. Forexample, in some embodiments, the concentration of B₂O₃ in the glasscomposition is greater than or equal to about 0 mol. % and less than orequal to about 0.5 mol. %, or even less than or equal to about 0.4 mol.%.

In the high boron glass compositions, B₂O₃ is generally present in theglass compositions in an amount greater than or equal to about 4.5 mol.%. In these embodiments, the impact of B₂O₃ on the HT CTE is mitigatedby controlling the ratio R₂O:Al₂O₃, as described above. For example, insome embodiments, B₂O₃ is present in the glass compositions in aconcentration greater than or equal to about 4.5 mol. % or even 5 mol. %and less than or equal to about 12 mol. %. In other embodimentsdescribed herein, the concentration of B₂O₃ in the glass composition isgreater than or equal to about 7 mol. % and less than or equal to about12 mol. %, or even greater than or equal to about 9 mol. % and less thanor equal to about 12 mol. %.

The glass compositions described herein may further include one or moredivalent oxides MO, where M is an alkaline earth metal (such as Mg orCa) and/or Zn. The divalent oxides improve the melting behavior of theglass compositions. Additions of MgO and ZnO also improve the ionexchange performance of the glass composition. In particular, it hasbeen found that additions of MgO and ZnO generally increase thecompressive stress and DOL for a given ion exchange condition (time andtemperature) without increasing the softening point of the glasscomposition. Additions of CaO to the glass composition generally assistin maintaining a sufficient DOL of the compressive stress following ionexchange strengthening.

In the low boron glass compositions described herein, the glasscompositions include at least one of MgO and ZnO and the totalconcentration of divalent oxide is greater than or equal to about 0.8mol. % or even 1 mol. % and less than or equal to about 10 mol. %.Moreover, in the low boron glass compositions described herein, the sumof the concentration of Al₂O₃ (mol. %) and the concentration of thedivalent oxide (mol. %) is generally greater than or equal to about 10mol. % which generally improves the ion exchange performance of theglass.

In the low boron glass compositions described herein, MgO may be presentin a concentration from about 0 mol. % to about 7 mol. %. For example,in some embodiments, the concentration of MgO may be greater than orequal to about 3 mol. % and less than or equal to about 5 mol. %. Insome other embodiments, the concentration of MgO may be greater than orequal to about 2 mol. % and less than or equal to about 4 mol. %.

Further, in the low boron glasses described herein, CaO may be presentin a concentration greater than or equal to about 0 mol. % and less thanor equal to about 1 mol. %. For example, in some embodiments, theconcentration of CaO may be greater than or equal to about 0 mol. % andless than or equal to about 0.5 mol. %.

In the low boron glass compositions described herein, ZnO may be presentin a concentration from about 0 mol. % to about 6 mol. %. For example,in some embodiments, the concentration of ZnO may be greater than orequal to about 2 mol. % and less than or equal to about 4 mol. %.

The high boron glass compositions described herein may also include oneor more divalent oxides MO. For example, in some embodiments of the highboron glass compositions which include MgO, the MgO may be present in aconcentration greater than or equal to about 0 mol. % and less than orequal to about 5 mol. %. In some embodiments of the high boron glasscompositions which include ZnO, the ZnO may be present in aconcentration greater than or equal to about 0 mol. % and less than orequal to about 5 mol. %. In some embodiments of the high boron glasscompositions which include CaO, the CaO may be present in aconcentration greater than or equal to about 0 mol. % and less than orequal to about 2 mol. %.

The glass compositions described herein (i.e. both low boron glasscompositions and high boron glass compositions) may also include P₂O₅.Additions of P₂O₅ increase the speed of ion exchange at a giventemperature such that an equivalent depth of layer may be reached in ashorter time period. In some embodiments of the glass compositionsdescribed herein, P₂O₅ may be present in the glass compositions in aconcentration greater than or equal to about 0 mol. % and less than orequal to about 3 mol. %. In some embodiments, the concentration of P₂O₅may be greater than or equal to about 0 mol. % and less than or equal toabout 2 mol. %. In some other embodiments, the concentration of P₂O₅ maybe greater than or equal to about 0.5 mol. % and less than or equal toabout 1.0 mol. %.

The glass compositions described herein may optionally include one ormore fining agents. The fining agents may include, for example, SnO₂,Sb₂O₃, As₂O₃, NaCl, (Al)OH₃, and CeO₂, and combinations thereof. Thefining agents may be present in the glass compositions in an amountgreater than or equal to about 0 mol. % and less than or equal to about1.0 mol. %. In exemplary embodiments, the fining agent is SnO₂. The SnO₂may be present in the glass composition in a concentration greater thanor equal to about 0 mol. % and less than or equal to about 1.0 mol. %.In some of these embodiments, SnO₂ may be present in the glasscomposition in a concentration which is greater than or equal to about 0mol. % and less than or equal to about 0.5 mol. % or even less than orequal to about 0.3 mol. %.

In some embodiments of the low boron glass compositions describedherein, the glass compositions may optionally comprise zirconia (ZrO₂).Additions of zirconia improve the ion exchange performance of the glasscomposition by increasing the depth of layer which may be achieved.However, if the amount of zirconia exceeds about 3 mol. %, the liquidusviscosity of the glass composition decreases, making the glasscomposition difficult to form. Accordingly, in the embodiments of theglass compositions which contain zirconia, the glass compositions mayinclude greater than or equal to about 0.5 mol. % and less than or equalto about 2 mol. % ZrO₂. In some of these embodiments, the low boronglass compositions may include greater than or equal to about 1.0 mol. %and less than or equal to about 1.5 mol. % ZrO₂. In some of theseembodiments, the concentration of ZrO₂ in the low boron glasscompositions is about 1 mol. %. However, it should be understood that,in some embodiments of the low boron glass compositions describedherein, the glass compositions are substantially free of zirconia(ZrO₂).

Certain applications of the glass compositions described herein mayrequire that the glass be opaque and have a certain color, such asblack. Accordingly, in the embodiments of the high boron glasscompositions described herein, the glass compositions may include one ormore constituents which act as a colorant. For example, some of the highboron glass compositions may include Fe₂O₃ and TiO₂ which, incombination, may be used to impart a black color and opacity to glassformed from the glass compositions. In these embodiments, Fe₂O₃ and TiO₂may be present in the glass composition such that a ratio of Fe₂O₃ (mol.%) to TiO₂ (mol. %) is in a range from greater than or equal to about0.52 to less than or equal to about 1.22. In some embodiments, the ratioof Fe₂O₃ (mol. %) to TiO₂ (mol. %) may be in a range from greater thanor equal to about 0.60 to less than or equal to about 1.00. As the ratioof Fe₂O₃ to TiO₂ decreases, the resultant glass article becomes lessblack as determined from the CIE F2 Illuminant and the L, a*, b* scale.In one particular embodiment, the total concentration of Fe₂O₃ and TiO₂(i.e., Fe₂O₃ (mol. %)+TiO₂ (mol. %)) is approximately 1.75 mol. %.However, it should be understood that other total concentrations ofFe₂O₃ and TiO₂ may also be utilized including total concentrationsgreater than 1.75 mol. % and total concentrations less than 1.75 mol. %.

In some embodiments described herein, a black color is achieved in theresultant glass formed from a glass composition containing colormodifying constituents by thermally treating the glass article. Forexample, in one embodiment, the glass may be initially annealed at atemperature in the range from about 560° C. to about 575° C. for a timeperiod of less than or equal to approximately 1 hour and subsequentlycooled. Thereafter, the glass may be heat treated at a temperature fromabout 600° C. to about 650° C. for less than or equal to approximately10 hours, after which the glass is opaque and has a black color. Whilenot wishing to be bound by theory, it is believed that the black coloris a result of precipitates formed from a combination of Fe₂O₃ and TiO₂(such as pseudo-brookite or the like) which precipitate in the glassduring thermal treatment.

In some embodiments, when the high boron glass composition includescolor modifying constituents such as those described above, the blackcolor following thermal treatment occurs when the alkali to aluminaratio (R₂O:Al₂O₃) is less than or equal to about 1.15 or even less thanor equal to about 1.12. In some embodiments, the black color may beobtained when the alkali to alumina ratio (R₂O:Al₂O₃) is greater than orequal to about 0.98 or even 1.02. For example, in some embodiments, theblack color may be obtained when the alkali to alumina ratio is lessthan or equal to about 1.15 and greater than about 0.98. In someembodiments, the black color is obtained when the alkali to aluminaratio is less than or equal to about 1.12 and greater than or equal toabout 1.02. In some other embodiments, the black color is obtained whenthe alkali to alumina ratio less than or equal to about 1.1 and greaterthan or equal to about 1.04.

In the embodiments of the high boron glass compositions which includecolor modifying constituents, the resultant glass may be opaque. Thedegree of opacity may be determined by the light absorption of the glassas measured by spectral photometry. In the embodiments described herein,the degree of opacity was measured by an X-Rite CI7 Spectro-Photometer.In the exemplary embodiments of the high boron glass with colormodifying agents described herein, the resultant opaque glass may havean opacity (i.e., a light absorption) greater than or equal to about 80%over a range of wavelengths from about 350 nm to about 750 nm incidenton the glass article. This means that less than about 20% of the lightincident on the glass article is actually transmitted through the glassarticle. In some embodiments, the opacity may be greater than or equalto about 80% and less than or equal to 100% over a range of wavelengthsfrom about 350 nm to about 750 nm. In some other embodiments, theopacity may be about 100% over a range of wavelengths from about 350 nmto about 750 nm.

As noted herein, color modifying agents added to the high boron glasscompositions may result in a glass which is black in color. The degreeof color may be quantified according using the CIE F2 Illuminant and theL, a*, b* scale. For example, in some embodiments, the glass article hasL, a*, b*, color coordinates in which L from about 0 to about 5.0; a*from about −2.0 to about 2.0; and b* from about 0 to about −5.0. Glasswith color coordinates within these ranges generally has a deep blackcolor.

In addition, it has been found that the high boron glass compositionswhich contain color modifying constituents as described herein arereadily amenable to strengthening by ion exchange. The depth of layer ofthese glass compositions may be greater than or equal to about 25 μm. Insome embodiments, the DOL may be greater than or equal to about 35 μm oreven greater than or equal to about 45 μm. It is believed that thecompressive stress imparted to these glass compositions by ion exchangemay be at least equal to or even greater than the same glass compositionwithout color modifying agents. However, measurement of the compressivestress using conventional techniques (such as stress birefringence) iscomplicated due to the optical properties of the glass (opacity andblack color). Accordingly, the characteristic strength of the glassarticle may be used as an estimate of the compressive stress imparted tothe glass. Specifically, ring-on-ring testing of a plurality ofun-abraded glass plate samples formed from the glass composition may betested using the methodology described in ASTM Standard C1499 entitled“Standard Test Method for Monotonic Equibiaxial Flexural Strength ofAdvanced Ceramics at Ambient Temperature.” From this data a Weibulldistribution of the strength at failure may be constructed and thecharacteristic strength and Weibull modulus may be determined. In theexemplary embodiments described herein, a glass plate formed from thehigh boron glass composition containing color modifying agents with athickness of approximately 0.8 mm generally has a characteristicstrength greater than or equal to about 1500 MPa or even about 1600 MPafollowing an anneal treatment at about 570° C. for approximately 2hours, a heat treatment at about 640° C. for approximately 4 hours andan ion exchange treatment at about 440° C. for approximately 15 hours ina salt bath of 100% KNO₃. In some embodiments, the characteristicstrength may be greater than or equal to about 1700 MPa or even about1800 MPa after the same treatments. In some other embodiments thecharacteristic strength may be greater than or equal to about 1900 MPaafter the same treatments. In the embodiments described herein, a glassplate formed from the high boron glass composition containing colormodifying agents with a thickness of approximately 0.8 mm generally hasa Weibull modulus greater than or equal to about 8 or even about 9following an anneal treatment at about 570° C. for approximately 2hours, a heat treatment at about 640° C. for approximately 4 hours andan ion exchange treatment at about 440° C. for approximately 15 hours ina salt bath of 100% KNO₃. In embodiments, the Weibull modulus may begreater than or equal to about 10 or even about 11 after the sametreatments. The Weibull modulus is the slope of the Weibull plot and isgenerally indicative of sensitivity of the material to failure due toflaws. In embodiments the characteristic strength may be greater than orequal to about 13 after the same treatments. The characteristic strengthis indicative of the strength at 63.2% the failure probability asdetermined by the Weibull distribution.

In addition to the relatively low softening points, HT CTEs, and ionexchange properties described above, the glass compositions describedherein also exhibit properties which make the glass compositionssuitable for use in fusion forming processes, such as the fusion downdraw process. Specifically, the glass compositions described herein haveliquidus temperatures of less than about 1000° C. and liquidusviscosities of greater than about 150 kP. Moreover, the glasscompositions also have zircon breakdown viscosities of less than about35 kP such that the glass compositions are compatible with fusionforming utilizing zirconia isopipes. Further, the glass compositionsdescribed herein also exhibit a viscosity of less than about 200 P atmelting temperatures in the range from about 1500° C. to about 1650° C.and viscosities of about 35 kP at forming temperatures from about 1050°C. to about 1150° C.

Based on the foregoing, it should be understood that various embodimentsof glass compositions with relatively low softening points andrelatively low high temperature coefficients of thermal expansion aredisclosed herein. In a first exemplary embodiment, a glass compositionincludes SiO₂, Al₂O₃, Li₂O and Na₂O. The glass composition may generallyhave a softening point less than or equal to about 810° C. and a hightemperature CTE less than or equal to about 27×10⁻⁶/° C. The glasscomposition may have a compressive stress greater than or equal to about650 MPa and a depth of layer greater than or equal to about 25 μm afterion exchange in a salt bath comprising KNO₃ at about 410° C. for lessthan or equal to approximately 15 hours.

In a second exemplary embodiment, a glass composition includes fromabout from about 65 mol. % to about 71 mol. % SiO₂; from about 7 mol. %to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O;from about 6 mol. % to about 16 mol. % Na₂O; from about 0 mol. % toabout 5 mol. % K₂O; from about 0.8 to about 10 mol. % of a divalentoxide, wherein the divalent oxide comprises at least one of MgO and ZnO;and less than about 0.5 mol. % B₂O₃, wherein a sum of the concentrationof Al₂O₃ (mol. %), and a concentration of the divalent oxide (mol. %) isgreater than about 10 mol %. In this second exemplary embodiment, theglass composition may optionally include from about 0.5 mol. % to about2 mol. % ZrO₂. Alternatively, this second exemplary composition may besubstantially free of ZrO₂. When the second exemplary compositions issubstantially free of ZrO₂, the glass composition may comprise fromabout 65.8 mol. % to about 71 mol. % SiO₂. The glass composition maygenerally have a softening point less than or equal to about 810° C. anda high temperature CTE less than or equal to about 27×10⁻⁶/° C. Theglass composition may have a compressive stress greater than or equal toabout 650 MPa and a depth of layer greater than or equal to about 25 μmafter ion exchange in a salt bath comprising KNO₃ at about 410° C. forless than or equal to approximately 15 hours.

In a third exemplary embodiment, a glass composition includes from aboutfrom about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % toabout 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃;from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In thisembodiment, R₂O is the sum of the concentration of Li₂O, theconcentration of Na₂O, and the concentration of K₂O, and the ratio ofR₂O to the concentration of Al₂O₃ is less than or equal to about 1.1.The glass composition may generally have a softening point less than orequal to about 810° C. and a high temperature CTE less than or equal toabout 27×10⁻⁶/° C. The glass composition may have a compressive stressgreater than or equal to about 650 MPa and a depth of layer greater thanor equal to about 25 μm after ion exchange in a salt bath comprisingKNO₃ at about 410° C. for less than or equal to approximately 15 hours.

In a fourth exemplary embodiment, a glass composition includes fromabout 65.8 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about6 mol. % to about 16 mol. % Na₂O; from about 1 mol. % to about 10 mol. %of a divalent oxide, wherein the divalent oxide is at least one of MgOand ZnO; and less than about 0.5 mol. % B₂O₃. The sum of theconcentration of Al₂O₃ (mol. %), and a concentration of the divalentoxide (mol. %) is greater than about 10 mol %. The glass composition maygenerally have a softening point less than or equal to about 810° C. anda high temperature CTE less than or equal to about 27×10⁻⁶/° C. In thisembodiment, the glass composition may be substantially free of ZrO₂.

In a fifth exemplary embodiment, a glass composition includes from about65.8 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6mol. % to about 16 mol. % Na₂O; and from about 1 mol. % to about 10 mol.% of a divalent oxide, wherein the divalent oxide is at least one of MgOand ZnO. The glass composition is substantially free from ZrO₂ and B₂O₃.The sum of the concentration of Al₂O₃ (mol. %), and the concentration ofthe divalent oxide (mol. %) is greater than 10 mol %. The glasscomposition may generally have a softening point less than or equal toabout 810° C. and a high temperature CTE less than or equal to about27×10⁻⁶/° C.

In a sixth exemplary embodiment, a glass composition includes from about67 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol.% Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. %to about 16 mol. % Na₂O; and from about 1 mol. % to about 10 mol. % of adivalent oxide, wherein the divalent oxide is at least one of MgO andZnO. The glass composition may also include from about 0 mol. % to about7 mol. % MgO; from about 0 mol. % to about 1 mol. % CaO; and from about0 mol. % to about 6 mol. % ZnO. The glass composition is substantiallyfree from B₂O₃. The sum of the concentration of Al₂O₃ (mol. %), and theconcentration of the divalent oxide (mol. %) is greater than about 10mol %. The glass composition may generally have a softening point lessthan or equal to about 810° C. and a high temperature CTE less than orequal to about 27×10⁻⁶/° C.

In a seventh exemplary embodiment, a glass composition includes fromabout from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. %to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃;from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In thisembodiment, R₂O is the sum of the concentration of Li₂O, theconcentration of Na₂O, and the concentration of K₂O, and the ratio ofR₂O to the concentration of Al₂O₃ is less than or equal to about 1.1.The glass composition may generally have a softening point less than orequal to about 810° C. and a high temperature CTE less than or equal toabout 27×10⁻⁶/° C.

In an eighth exemplary embodiment, a glass composition includes fromabout 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. %Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In this embodiment,R₂O is the sum of the concentration of Li₂O, the concentration of Na₂O,and the concentration of K₂O, and the ratio of R₂O to the concentrationof Al₂O₃ is less than or equal to about 1.1. The glass composition maygenerally have a softening point less than or equal to about 810° C. anda high temperature CTE less than or equal to about 27×10⁻⁶/° C.

In a ninth exemplary embodiment, a glass composition includes from about55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol.% Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol.% to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O;and from about 0 mol. % to about 3 mol % K₂O. The glass composition mayfurther include from about 0 mol. % to about 5 mol. % MgO; from about 0mol. % to about 5 mol. % ZnO; and from about 0 mol. % to about 2 mol. %CaO. In this embodiment, R₂O is the sum of the concentration of Li₂O,the concentration of Na₂O, and the concentration of K₂O and the ratio ofR₂O to the concentration of Al₂O₃ is less than or equal to about 1.1.The glass composition may generally have a softening point less than orequal to about 810° C. and a high temperature CTE less than or equal toabout 27×10⁻⁶/° C.

In a tenth exemplary embodiment, a glass composition includes from about65 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol.% Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. %to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. % K₂O; fromabout 0.8 to about 10 mol. % of a divalent oxide, wherein the divalentoxide comprises at least one of MgO and ZnO; from about 0.5 mol. % toabout 2 mol. % ZrO₂; and less than about 0.5 mol. % B₂O₃. In thisexemplary embodiment, the sum of the concentration of Al₂O₃ (mol. %),and the concentration of the divalent oxide (mol. %) is greater thanabout 10 mol %. The glass composition has a softening point less than orequal to about 810° C. The glass composition has a high temperature CTEless than or equal to about 27×10⁻⁶/° C.

Examples

The embodiments of the glass compositions described herein will befurther clarified by the following examples.

A plurality of exemplary glass compositions were prepared according tothe batch compositions listed in Tables 1-6 below. Batches of the oxideconstituent components were mixed, melted and formed into glass. Theproperties of the glass composition (i.e., softening point, HT CTE,etc.) were measured and the results are reported in Tables 1-6.Comparative Examples (i.e., non-inventive Examples) are prefaced with a“C” in the example identification.

Referring now to Table 1, seven exemplary low boron glass compositionswere prepared to investigate the effect of substituting Li₂O for Na₂O onsoftening point and HT CTE. FIG. 2 graphically depicts the softeningpoint (y-axis) as a function of the concentration of Li₂O (x-axis) asLi₂O is substituted for Na₂O. FIG. 3 graphically depicts the HT CTE(y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O issubstituted for Na₂O.

As indicated by the data in Table 1 and FIGS. 2 and 3, the partialsubstitution of Li₂O for Na₂O resulted in a progressive decrease in thesoftening point of the glass composition. In particular, substitution of9 mol. % Li₂O for Na₂O decreased the softening point by up to 70° C. Thesubstitution of Li₂O for Na₂O also caused a slight increase in the HTCTE (from 24×10⁻⁶/° C. to 25.5×10⁻⁶/° C. for a 1-3 mol. % substitution).However, even after the substitution, the HT CTE values of thesecompositions were less than 27×10⁻⁶/° C. Moreover, as graphicallydepicted in FIG. 3, the data from Table 1 indicates that the increase inHT CTE levels off for increasing concentrations of Li₂O.

TABLE 1 Substitution of Li₂O for Na₂O (Mol % ) 1 2 3 4 5 6 7 SiO₂ 69.469.4 69.4 69.4 69.4 69.4 69.4 Al₂O₃ 11.2 11.2 11.2 11.2 11.2 11.2 11.2B₂O₃ 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Na₂O 14.2 13.2 11.2 10.2 9.2 7.7 6.2K₂O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 Li₂O 1.0 2.0 4.0 5.0 6.0 7.5 9.0 CaO 0.30.3 0.3 0.3 0.3 0.3 0.3 MgO 0.3 0.3 0.3 0.3 0.3 0.3 0.3 ZnO 0.3 0.3 0.30.3 0.3 0.3 0.3 SnO₂ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Softening pt, ° C. 792778 751 751 748 742 732 HT CTE, ppm/° C. 24.4 25.3 25.7 25.1 25 25.325.3

Referring now to Table 2, eight exemplary low boron glass compositionswere prepared to investigate the effect of substituting Li₂O for Na₂Oand Na₂O+K₂O on softening point and HT CTE. FIG. 4 graphically depictsthe softening point (y-axis) as a function of the concentration of Li₂O(x-axis) as Li₂O is substituted for Na₂O+K₂O. FIG. 5 graphically depictsthe HT CTE (y-axis) as a function of the concentration of Li₂O (x-axis)as Li₂O is substituted for Na₂O+K₂O.

As indicated by the data in Table 2 and FIGS. 4 and 5, the partialsubstitution of Li₂O for Na₂O or Na₂O+K₂O resulted in a progressivedecrease in the softening point of the glass composition. In particular,substitution of 5 mol. % Li₂O for Na₂O+K₂O decreased the softening pointby up to about 90° C. The substitution of Li₂O for Na₂O+K₂O also causeda slight increase in the HT CTE. However, even after the substitution,the HT CTE values of these compositions were less than 27×10⁻⁶/° C. Noleveling of the HT CTE was observed as with the glass compositions ofTable 2.

TABLE 2 Substitution of Li₂O for Na₂O and K₂O (Mol %) C1 C2 8 9 10 11 1213 SiO₂ 69.8 69.8 69.8 69.8 69.8 69.8 69.8 69.8 Al₂O₃ 9.0 9.0 9.0 9.09.0 9.0 9.0 9.0 Li₂O 1.0 1.5 3.0 3.5 4.9 4.9 4.9 5.0 Na₂O 12.7 12.2 10.710.2 10.0 10.0 10.0 8.7 K₂O 1.7 1.7 1.7 1.7 0.7 0.7 0.7 1.7 MgO 5.1 5.15.1 5.1 5.1 5.1 5.1 5.1 CaO 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 SnO₂ 0.2 0.20.2 0.2 0.3 0.3 0.2 0.2 total 15.4 15.4 15.4 15.4 15.6 15.6 15.6 15.4R₂O softening 833 822 799 791 770 779 781 777 pt HT CTE 22.3 21.8 22.523.3 24.3 23.8 23.7 23.8

Table 3 contains the post-ion exchange properties (compressive stressand DOL) for several of the exemplary glass compositions listed in Table2 for a variety of different ion exchange processing conditions. The ionexchange conditions included single ion exchange treatments in 100% KNO₃at a variety of temperature and immersion times, as well as two steptreatments involving a mixed bath (e.g., 90% KNO₃-10% NaNO₃), followedby immersion in a second bath of 100% KNO₃. The compressive stress andDOL values shown in Table 3 are measured values obtained with a FSMinstrument, with the compressive stress value being based on themeasured stress optical coefficient (SOC). FIG. 6 graphically depictsthe compressive stress (y-axis) and DOL (x-axis) for different Li₂Oconcentrations (i.e., 1.0 mol. %, 3.5 mol. %, and 4.9 and 5.0 mol. %(collectively)). As shown in FIG. 6, a 40 μm DOL can be achieved for the1% and 3.5% Li₂O concentrations, but not for greater concentrations,thereby indicating that, at least for Li-for-Na substitution, there maybe an effective upper limit to the amount of Li₂O that can be introducedinto the glass while still achieving the desired DOL.

TABLE 3 Ion Exchange Properties of Select Compositions 9 11 12 13 Li₂O(mole %)  3.5  4.9  4.9  5.0 IX Schedule, CS (MPa), DOL (um) 410-8 h 100KNO₃ 836, 19 820, 19 805, 21 410-10 h 100 KNO₃ 803, 24 816, 18 806, 26410-12 h 100 KNO₃ 783, 25 803, 23 410-15 h 100 KNO₃ 801, 28 775, 26420-6 h 100 KNO₃ 782, 24 803, 18 420-8 h 100 KNO₃ 772, 27 773, 21 762,22 420-15 h 100 KNO₃ 765, 32 430-15 h 100 KNO₃ 768, 36 707, 37 440-15100 KNO₃ 715, 42 390-15 h 80 KNO₃/20 NaNO₃ 853, 19 886, 14 904, 16 +430-1 h 100 KNO₃ 390-15 h 90 KNO₃/10 NaNO₃ 889, 14 897, 16 710, 19 +430-1 h 100 KNO₃ 410-15 h 80 KNO₃/20 NaNO₃ 759, 24 897, 18 882,18 710,22 + 430-1 h 100 KNO₃ 410-15 h 90 KNO₃/10 NaNO₃ 759, 27 687, 25 + 649,24 430-1 h 100 KNO₃

Referring now to Table 4, Table 4 contains the compositional data andcorresponding softening points, HT CTEs, and ion exchangecharacteristics of several inventive and comparative examples of lowboron glass compositions.

TABLE 4 Low Boron Glass Compositions (Mo1%) C3 C4 14 15 16 17 18 19 2021 22 C5 SiO₂ 65.8 65.8 67.8 67.8 65.8 69.4 69.4 69.4 69.6 68.2 67.365.9 Al₂O₃ 10.0 10.0 10.0 10.0 8.0 11.2 10.7 10.7 7.5 7.3 9.0 9.0 Li₂O4.0 3.0 3.0 3.0 3.0 3.0 1.0 3.0 2.1 2.1 3.5 3.5 Na₂O 12.0 13.0 15.0 15.013.0 15.4 15.2 13.2 14.1 13.4 14.6 14.6 K₂O 0 0 0 0 0 0 2.0 2.0 0 2.00.5 0.5 MgO 4.0 4.0 2.0 0 5.0 0.5 0.5 0.5 6.6 6.4 5.1 6.5 CaO 0 0 0 0 00 0 0 0 0.5 0 0 ZnO 4.0 4.0 2.0 4.0 5.0 0.5 0.5 0.5 0 0 0 0 P₂O₅ 0 0 0 00 0 0.8 0.8 0 0 0 0 SnO₂ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2MgO + ZnO 8.0 8.0 4.0 4.0 10.0 1.0 1.0 1.0 6.6 6.4 5.1 6.5 Al₂O₃ + MgO +ZnO 18.0 18.0 14.0 14.0 18.0 12.2 11.7 11.7 14.1 13.7 14.1 15.5Softening pt, ° C. 778 785 757 764 752 775 802 785 781 747 750 742 HTCTE, ppm/° C. 24.0 21.8 24.2 24.3 24.0 22.7 23.2 23.9 25.6 26.6 25.6 IX,430-15 h (100 KNO₃) CS (MPa) 868 950 637 872 DOL (μm) 31 32 43 28 IX,430-15 h (100 KNO₃) CS (MPa) 796 893 587 DOL (μm) 36 37 50 IX, 410-15 h(90 KNO₃/10 NaNO₃) CS (MPa) 690 720 662 760 DOL (μm) 62 48 38 34

Referring now to Table 5, a series of low boron glass compositioncontaining zirconia were also formed. The softening points of theseglass compositions were less than 800° C. and the HT CTEs were less than27×10⁻⁶/° C. Following ion exchange in a salt bath of 100% KNO₃ at 440°C. for 15 hours the glass compositions had a compressive stress greaterthan 700 MPa and a depth of layer of 40 μm.

TABLE 5 Glass Compositions with Zirconia Additions (Mol %) 23 24 25 2627 28 SiO₂ 65.9 65.4 65.9 65.9 65.9 64.9 Al₂O₃ 9 9 9 8 8.5 8.5 Li₂O 3.53.5 3.5 3.5 3.5 3.5 Na₂O 14.6 14.6 16.1 16.1 16.1 16.1 K₂O 0.5 0.5 0.50.5 0.5 0.5 MgO 5.5 3 3.5 3.5 3 3 ZnO 0 3 0 0.5 1 2 P₂O₅ 0 0 0 1 0 0ZrO₂ 1 1 1.5 1 1.5 1.5 SnO₂ 0.2 0.2 0.2 0.2 0.2 0.2 Soft pt (° C.) 771758 752 751 744 744 HT CTE (ppm) 24.6 24.4 25.0 26.1 25.4 25.3 IX (100%665,43 721,42 505,49 440,63 489,49 523,46 KNO₃ @ 440° C. for 15 hrs)CS,DOL

Referring to Table 6, two inventive compositions (29 and 30) weresubjected to additional characterization and a determination of whetherthe glass compositions would be suitable for use with fusion formingprocesses. Table 6 contains the composition and properties of theinventive compositions as well as two comparative examples of ionexchangeable glass compositions suitable for use with a fusion formingprocess. The inventive low boron glass compositions had softening pointsthat are approximately 40° C. lower than the softening points of thecomparative glass compositions. Further, the HT CTE values of the lowboron glass compositions are comparable to or less than the HT CTEvalues of the comparative glass compositions. The inventive low boronglass compositions have similar ion exchange properties as well as hightemperature viscosities, liquidus temperatures, liquidus viscosities andzircon breakdown temperatures as the comparative glass compositionsindicating that the inventive low boron glass compositions would besuitable for use with fusion forming processes.

TABLE 6 Forming Characteristics of Inventive and Comparative GlassCompositions Mol % C6 C7 29  30 SiO₂  66.0  69.2  69.8  66.8 Al₂O₃  10.3  8.5   9.2  10.0 B₂O₃   0.6   0   0   0 Li₂O   0   0   3.5   3.0 Na₂O 14.2  13.9  11.9  14.0 K₂O   2.4   1.2   0.5   0 MgO   5.8   6.5   5.1  3.0 CaO   0.6   0.5   0   0 ZnO   0   0   0   3.0 SnO₂   0.2   0.2  0.2   0 soft pt  837  844  799  781 HT CTE  23.0  21.0  22.6  24.0 DOL 40  40  40  44 CS  740  740  680  785 HTV: 200 P 1588 1640 1609 1567HTV: 35 kP 1131 1145 1117 1084 delta  457  495  492  483 Liquidus temp 900  950  965  970° Liquidus >1e6 >1e6  500 kP  275 kP viscosityviscosity (RT)   2.461   2.444   2.438   2.497

Referring now to Table 7, Table 7 contains the compositional data andcorresponding softening points and HT CTEs for inventive and comparativehigh boron glass compositions. As shown in Table 7, the inventive glasscompositions generally have greater than about 5 mol. % and, in theexamples shown, up to about 10 mol % B₂O₃. However, for the inventivehigh boron glass compositions shown in Table 7, the ratio R₂O:Al₂O₃ isgenerally greater than or equal to about 0.9 and less than or equal toabout 1.15.

Comparative Example C8 does not contain Li₂O and, as a result, thesoftening point for this glass is greater than the inventive high boronglass compositions illustrating the need to have Li₂O in the glasscompositions.

TABLE 7 High Boron Glass Compositions (Mol %) C8 31 32 33 34 35 36 37SiO₂ 64.2 60 58.6 65.7 65.7 65.7 65.2 63.7 Al₂O₃ 12.0 14.4 13.8 12.312.3 11.3 12.3 12.3 B₂O₃ 9.0 5.2 5.1 9.1 9.1 9.1 9.1 9.1 Li₂O 0 6.1 4.95.0 7.0 5.0 5.0 5.0 Na₂O 11.3 7.6 8.2 6.6 4.6 6.6 6.6 6.6 K₂O 0.5 0.50.5 1.3 1.3 1.3 1.3 1.3 MgO 1.5 2.7 2.7 0 0 1 0 0 CaO 0 0 0 0 0 0 0 0ZnO 1.5 3.5 3.9 0 0 0 0 0 P₂O₅ 0 0 2.3 0 0 0 0.5 2 SnO₂ 0.3 0.3 0.3 0.00.0 0.1 0.1 0.1 R₂O/Al₂O₃ 0.98 0.99 0.99 1.05 1.05 1.14 1.05 1.05softening pt. 845 770 781 785 786 745 795 789 HT CTE 24.6 26.7 25.4 26.731.4 25.1 26.0

Referring now to Table 8, five glass compositions were formulatedcontaining the color modifying constituents TiO₂ and Fe₂O₃ to produce anopaque, black glass. Inventive compositions 38-41 were based oncomposition 31 from Table 7 and inventive composition 42 was based oncomposition 34 of Table 7. The alkali to alumina ratio of each glasscomposition was less than 1.15 and greater than 1.0. Plate sampleshaving a thickness of 0.8 mm were produced and measured. Each ofcompositions 39-42 exhibited a softening point of less than about 810°C. with composition 38 having a softening point of approximately 814° C.which is about 810° C. given the measurement error (+/−5° C.) of theinstrument. Compositions 38 and 41-42 also exhibited an HT CTE of lessthan or equal to about 27×10⁻⁶/° C. (the HT CTE for compositions 39 and40 was not measured). Each sample was annealed at 570° C. for 2 hours,cooled to room temperature and heat treated at 640° C. for 4 hours toproduce the black color. The opacity of the samples was qualitativelyassessed by shining a light source on the surface of the plate andqualitatively determining if the light was transmitted through the otherside of the plate. As indicated in Table 8, all samples appeared opaquebased on this qualitative assessment. In addition the L, a*, b*,coordinates were determined for compositions 38, 41, and 42 using anX-Rite CI7 Spectro-Photometer. Samples of compositions 41 and 42 werealso ion exchanged in a 100% KNO₃ salt bath at 430° C. for 15 hours. Itwas determined that the resultant depth of layer of potassium diffusionin each sample was greater than 30 μm.

TABLE 8 High Boron Glass Compositions with Color Modifying Constituents(Mol %) 38 39 40 41 42 SiO₂  61.0 62.0  62.0  63.5  65.7 Al₂O₃  13.413.8  13.9  13.9  12.3 B₂O₃  5.2  5.2  5.2  5.2  9.1 Li₂O  3.5  3.5  3.7 3.7  3.5 Na₂O  10.2 10.7  10.7  10.7  8.9 K₂O  0.5  0.5  0  0.5  0.5MgO  2.7  3.3  3.0  2.0  0 ZnO  3.5  1.0  1.0  0.5  0.0 TiO₂  1.0  1.0 1.0  1.0  1.0 Fe₂O₃  0.75  0.75  0.75  0.75  0.75 SnO₂  0.2  0.1  0.1 0  0.1 R₂O/Al₂O₃  1.06  1.07  1.04  1.07  1.05 Soft pt, ° C. 814 794799 802 HT CTE, ppm/° C.  21.7  27.0  24.8 Opaque to snake light? yesyes yes yes yes (0.8 mm thick; 570° anneal) Color coordinates (CIE F2,10° diffuse, reflectance), 0.8 mm thick (570° anneal) L  0.20  0.785 0.54 a*  −0.50  −0.21  0.06 b*  −0.11  −1.08  −0.16 IX, 430°-15 hr 100 35  36 KNO₃, DOL (um)

Referring now to Table 9, a series of glass compositions were formulatedto assess the effects of the alkali (R₂O) to alumina (Al₂O₃) ratio onforming black glasses. As shown in Table 9, 6 glass compositions wereformulated with increasing R₂O:Al₂O₃ ratios. The glass compositions wereformed into 0.8 mm glass plates, annealed at 570° C. for 2 hours, cooledto room temperature, and heat treated at 640° C. for 4 hours.Comparative compositions C9 and C10 did not result in an opaque glassusing the qualitative assessment described above. Moreover, thesoftening points of these samples were much greater than 810° C.Comparative composition C12 did not result in an opaque glass using thequalitative assessment described above and the HT CTE of the sample wasgreater than 27×10⁻⁶/° C. While comparative composition C11 resulted inan opaque glass, the HT CTE of the sample was greater than 27×10⁻⁶/° C.Inventive composition 43 and 44 both yielded opaque glasses using thequalitative assessment described above and both had HT CTEs less than27×10⁻⁶/° C. The softening point of inventive composition 43 wasmeasured at 812° C. which is approximately 810° C. given the measurementerror (+/−5° C.) of the instrument.

TABLE 9 High Boron Glass Compositions With Varying R₂O:Al₂O₃ Ratios (Mol%) C9 C10 43 44 C11 C12 SiO₂ 63.5 63.5 63.5 63.5 63.5 63.5 Al₂O₃ 14.714.4 14.1 13.85 13.6 13.2 B₂O₃ 5.2 5.2 5.2 5.2 5.2 5.2 Li₂O 3.5 3.6 3.73.7 3.8 3.9 Na₂O 10.2 10.4 10.6 10.8 10.9 11.2 K₂O 0.4 0.4 0.4 0.45 0.50.5 MgO 2.0 2.0 2.0 2.0 2.0 2.0 ZnO 0.5 0.5 0.5 0.5 0.5 0.5 TiO₂ 1.0 1.01.0 1.0 1.0 1.0 Fe₂O₃ 0.75 0.75 0.75 0.75 0.75 0.75 R₂O/Al₂O₃ 0.96 1.001.04 1.08 1.12 1.18 Soft pt, ° C. 837 824 812 804 794 776 HT CTE. ppm/°C. 24.6 25.5 26.5 26.8 27.4 31.1 Opaque to no no yes yes yes no snakelight? (0.8 mm thick)** Color coordinates (CIE F2, 10° diffuse,reflectance), 0.8 mm thick* L 4.41 4.56 4.01 4.25 4.21 7.99 a* −1.21−1.23 −1.08 −1.19 −1.17 −1.52 b* −4.92 −4.93 −4.54 −4.77 −4.78 −6.14**(570-2 hr anneal + 640-4 hr heat-treat)

Referring now to Table 10, glass plates formed from composition 41 ofTable 8 were prepared and heat treated under different conditions. Someof these samples were also ion exchanged to assess the benefit ofstrengthening glass formed from a glass composition containing colormodifying constituents. Specifically, a first set of 17 plates formedfrom inventive composition 41 and having a thickness of 0.83 mm wereannealed at 570° C. for 2 hours. A second set of 17 plates formed frominventive composition 41 and having a thickness of 0.83 mm were annealedat 570° C. for 2 hours, cooled to room temperature, and then ionexchanged in a salt bath of 100% KNO₃ at 440° C. for 15 hours. A thirdset of 12 plates formed from inventive composition 41 and having athickness of 0.80 mm were annealed at 570° C. for 2 hours, cooled toroom temperature, and then heat treated at 640° C. for 4 hours. A fourthset of 15 plates formed from inventive composition 41 and having athickness of 0.80 mm were annealed at 570° C. for 2 hours, cooled toroom temperature, heat treated at 640° C. for 4 hours, and then ionexchanged in a salt bath of 100% KNO₃ at 440° C. for 15 hours. Theconcentration of sodium and potassium ions for one plate as a functionof depth into the plate is graphically depicted in FIG. 7 indicatingthat the depth of layer induced by ion exchange was approximately 30Each of the plates was tested in un-abraded condition using aring-on-ring testing protocol according to ASTM Standard C1499 in orderto assess the degree of strengthening achieved by ion exchange. Forpurposes of comparison, 15 glass plates formed from Corning glass code2318 (ion exchange strengthened borosilicate glass sold by Corning Inc.under the trademark Gorilla Glass™) were tested according to the sametesting protocol. A Weibull distribution for each set of glass plateswas constructed and the characteristic strength and Weibull modulus weredetermined.

As shown in Table 10, the inventive glass composition 41 had asignificant increase in strength following ion exchange in both theannealed condition and the annealed and heat treated condition. Thisresulted in a degree of strengthening similar to that of found inCorning glass code 2318.

TABLE 10 Ring-On-Ring Testing Strength At Failure Number Strength atFailure Weibull Thickness of (Mean ± 1 Characteristic modulus Treatment(mm) Samples S.D. (% CV)) Strength (So) (m) Annealed (570-2 h), NIX 0.8317  412 ± 62 (14.9%)  438  7.5 Annealed (570-2 h), 0.83 17 1852 ± 146(7.9%) 1920 13.8 IX (440-15 h, 100 KNO3) Annealed (570-2 h), 0.80 12 317 ± 85 (26.8%)  352  3.7 Heat-treated (640-4 h), NIX Annealed (570-2h), 0.80 15 1763 ± 151 (8.6%) 1832 12.5 Heat-treated (640-4 h), IX(440-15 h, 100 KNO3) Control - 2318 IX 1.00 15 1390 ± 320 (23.0%) 1518 4.1

It should now be understood that the glass compositions described hereinare suitable for use in conjunction with elevated temperature formingprocesses for shaping the glass compositions into 3-D shaped glassarticles. Specifically, the relatively low softening points of the glasscompositions described herein (i.e., softening points of less than orequal to about 810° C.) decrease the interaction between the mold andthe glass composition during elevated temperature shaping therebyimproving the formability of the glass composition and also increasingthe service life of the corresponding mold.

Further, the glass compositions described herein also exhibit arelatively low high temperature coefficient of thermal expansion abovethe glass transition temperature (i.e., an HT CTE of less than or equalto about 27×10⁻⁶/° C.). The relatively low HT CTE provides the glasscompositions good dimensional control of the glass composition followingelevated temperature forming processes.

While the glasses described herein have relatively low softening pointsand relatively low HT CTEs, the glass compositions are also ionexchangeable. For example, the glass compositions described herein maybe ion exchange strengthened to achieve a depth of layer of greater thanor equal to about 25 μm and a compressive stress of about 650 MPafollowing immersion in a molten salt bath comprising KNO₃ at about 410°C. for less than or equal to approximately 15 hours.

Further, the glass compositions described herein have liquidusviscosities and viscosities lower than about 200 P at about 1620° C.such that the glass compositions are compatible with fusion drawprocesses and are easily melted.

It should now be understood that several aspects of glass articles andglass compositions are disclosed herein. In a first aspect, a glassarticle comprising SiO₂, Al₂O₃, Li₂O and Na₂O has a softening point lessthan or equal to about 810° C.; a high temperature CTE less than orequal to about 27×10⁻⁶/° C.; and a compressive stress greater than orequal to about 600 MPa and a depth of layer greater than or equal toabout 25 μm after ion exchange in a salt bath comprising KNO₃ in atemperature range from about 390° C. to about 450° C. for less than orequal to approximately 15 hours.

In a second aspect, the glass article of the first aspect has L, a*, b*,color coordinates of L from about 0 to about 5.0, a* from about −2.0 toabout 2.0, and b* from about 0 to about −5.0.

In a third aspect, the glass article of any of the first or secondaspects has an opacity greater than or equal to about 80% over a rangeof wavelengths from about 350 nm to about 750 nm.

In a fourth aspect, the glass article of any of the first through thirdaspects comprises from about 65 mol. % to about 71 mol. % SiO₂; fromabout 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 0mol. % to about 5 mol. % K₂O; from about 0.8 to about 10 mol. % of adivalent oxide, wherein the divalent oxide comprises at least one of MgOand ZnO; and less than about 0.5 mol. % B₂O₃, wherein a sum of aconcentration of Al₂O₃ (mol. %), and a concentration of the divalentoxide (mol. %) is greater than about 10 mol %.

In a fifth aspect, the glass article of the fourth aspect furthercomprises from about 0.5 mol. % to about 2.0 mol. % ZrO₂.

In a sixth aspect, the glass article of any of the fourth or fifthaspects comprises from about 0 mol. % to about 3 mol. % P₂O₅.

In a seventh aspect, the glass article of any of the fourth throughsixth aspects is substantially free of ZrO₂.

In an eighth aspect, the glass article of the seventh aspect comprisesfrom about 65.8 mol. % to about 71 mol. % SiO₂.

In a ninth aspect, the glass article of any of the first through thirdaspects comprises from about 55 mol. % to about 68 mol. % SiO₂; fromabout 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol %K₂O. In this ninth aspect, R₂O is a sum of a concentration of Li₂O, aconcentration of Na₂O, and a concentration of K₂O, and a ratio of R₂O toa concentration of Al₂O₃ is less than or equal to about 1.15.

A tenth aspect includes the glass article of the ninth aspect whereinthe ratio of R₂O to the concentration of Al₂O₃ is greater than or equalto about 1.02.

In an eleventh aspect, the glass article of any of the ninth or tenthaspects further comprises TiO₂ and Fe₂O₃.

A twelfth aspect includes the glass article of the eleventh aspect,wherein a ratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) is greater than orequal to about 0.52 and less than or equal to about 1.22.

In a thirteenth aspect, a glass composition includes from about 65.8mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. %Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. %to about 16 mol. % Na₂O; from about 0.8 to about 10 mol. % of a divalentoxide, wherein the divalent oxide is at least one of MgO and ZnO; andless than about 0.5 mol. % B₂O₃. A sum of a concentration of Al₂O₃ (mol.%) and a concentration of the divalent oxide (mol. %) is greater thanabout 10 mol %. The glass composition has a softening point less than orequal to about 810° C. The glass composition also has a high temperatureCTE less than or equal to about 27×10⁻⁶/° C. This glass composition maybe substantially free from ZrO₂.

A fourteenth aspect includes the glass composition of the thirteenthaspect, wherein the glass composition is substantially free of B₂O₃.

In a fifteenth aspect, the glass composition of any of the thirteenththrough fourteenth aspects has a high temperature CTE is less than orequal to about 25×10⁻⁶/° C.

In a sixteenth aspect, the glass composition of any of the thirteenththrough fourteenth aspects further includes from about 0 mol. % to about7 mol. % MgO; from about 0 mol. % to about 1 mol. % CaO; and from about0 mol. % to about 6 mol. % ZnO.

In a seventeenth aspect, the glass composition of the sixteenth aspecthas a concentration of MgO is greater than or equal to about 3 mol. %and less than or equal to about 5 mol. %.

In an eighteenth aspect, the glass composition of any of the thirteenththrough seventeenth aspects includes a concentration of SiO₂ from about68 mol. % to about 71 mol. %.

In a nineteenth aspect, a glass composition includes from about 55 mol.% to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃;from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % toabout 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; andfrom about 0 mol. % to about 3 mol % K₂O. In this aspect, R₂O is a sumof a concentration of Li₂O, a concentration of Na₂O, and a concentrationof K₂O, and a ratio of R₂O to a concentration of Al₂O₃ is less than orequal to about 1.15. The glass composition may have a softening pointless than or equal to about 810°. The glass composition may also have ahigh temperature CTE less than or equal to about 27×10⁻⁶/° C.

A twentieth aspect includes the glass composition of the nineteenthaspect, wherein the softening point is less than or equal to about 800°C.

A twenty-first aspect includes the glass composition of any of thenineteenth through twentieth aspects wherein the high temperature CTE isless than or equal to about 25×10⁻⁶/° C.

A twenty-second aspect includes the glass composition of any of thenineteenth through twentieth aspects wherein the ratio of R₂O to theconcentration of Al₂O₃ is greater than or equal to about 1.02 and theglass composition further comprises TiO₂ and Fe₂O₃.

A twenty-third aspect includes the glass composition of any of thenineteenth through twenty-second aspects wherein a ratio of Fe₂O₃ (mol.%) to TiO₂ (mol. %) is greater than or equal to about 0.52 and less thanor equal to about 1.22.

A twenty-fourth aspect includes the glass composition of any of thenineteenth through twenty-third aspects, wherein the glass compositionfurther comprises from about 0 mol. % to about 5 mol. % MgO; from about0 mol. % to about 5 mol. % ZnO; and from about 0 mol. % to about 2 mol.% CaO.

A twenty-fifth aspect includes the glass composition of any of thenineteenth through twenty-fourth aspects, wherein a concentration ofB₂O₃ is greater than or equal to 7 mol. % and less than or equal to 12mol. %.

A twenty-sixth aspect includes the glass composition of any of thenineteenth through twenty-fifth aspects, wherein the glass compositioncomprises from about 0 mol. % to about 3 mol. % P₂O₅.

A twenty-seventh aspect includes a glass composition comprising fromabout 65 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6mol. % to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. %K₂O; from about 0.8 to about 10 mol. % of a divalent oxide, wherein thedivalent oxide comprises at least one of MgO and ZnO; from about 0 mol.% to about 3 mol. % P₂O₅. from about 0.5 mol. % to about 2 mol. % ZrO₂;and less than about 0.5 mol. % B₂O₃. In this aspect, a sum of aconcentration of Al₂O₃ (mol. %), and a concentration of the divalentoxide (mol. %) is greater than about 10 mol %. The glass composition mayhave a softening point less than or equal to about 810° C. The glasscomposition may also have a high temperature CTE less than or equal toabout 27×10⁻⁶/° C.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A glass article comprising SiO₂, Al₂O₃, Li₂O andNa₂O, the glass article having: a softening point less than or equal toabout 810° C.; a high temperature CTE less than or equal to about27×10⁻⁶/° C.; and a compressive stress greater than or equal to about600 MPa and a depth of layer greater than or equal to about 25 μm afterion exchange in a salt bath comprising KNO₃ in a temperature range fromabout 390° C. to about 450° C. for less than or equal to approximately15 hours.
 2. The glass article of claim 1 comprising: from about 55 mol.% to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃;from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % toabout 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; andfrom about 0 mol. % to about 3 mol % K₂O, wherein: R₂O is a sum of aconcentration of Li₂O, a concentration of Na₂O, and a concentration ofK₂O, and a ratio of R₂O to a concentration of Al₂O₃ is less than orequal to about 1.15.
 3. The glass article of claim 2, wherein the ratioof R₂O to the concentration of Al₂O₃ is greater than or equal to about1.02.
 4. The glass article of claim 3, wherein the glass article furthercomprises TiO₂ and Fe₂O₃.
 5. The glass article of claim 4, wherein aratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) is greater than or equal toabout 0.52 and less than or equal to about 1.22.